It is well known that plastic pipes can be made by helically winding a plastic strip having a series of spaced apart upstanding ribs extending longitudinally of the strip, either at room temperature or at an elevated temperature where the plastic becomes more flexible. This form of helically wound tube is already well known in the piping industry and is described in patents by the applicant relating both to the form of the plastic strip and the form of the machine by means of which the pipes or tubes are produced from such strips.
For these pipes to perform in a high performance applications, in order to attain the necessary degree of strength, the wall thickness of the plastic strip must be quite substantial, as well as that of the ribs. Alternatively the finished pipes or tubes can be reinforced with strengthening or reinforcing members.
In applications where the reinforced tubes or pipes are buried in a trench or subjected to high earth loads, the strength of the pipe or tube is of extreme importance.
The applicant's Australian Patent No. 607431 discloses a method of producing a reinforced plastics tube utilising a reinforcing member placed between the ribs in such a manner that the deflection resistance of the finished pipe or tube is materially increased. The reinforcing member comprises a metal member having a profile of U-shape cross-section, the free ends of the reinforcing member being designed to engage beneath opposed flange formations of a pair of adjacent ribs to thereby lock the metal strip in position between the ribs and in turn stiffen the ribs and the finished pipe.
The applicant's Australian Patent No. 661047 discloses an improvement over the disclosure of Australian Patent No. 607431 referred to above. The improvement is provided by the provision of a reinforcing member having a central body portion of inverted U or V-shaped cross-section that has a radial height greater than the height of the ribs whereby the effective external diameter of the composite pipe is substantially increased. This provides a stiffer pipe.
Known helically wound composite pipes are formed in a multi-stage operation. The plastic body is extruded and then is helically wound to form a pipe. Elongate steel reinforcing members are separately roll-formed into a profile providing the required stiffness (such as the inverted U or V-shaped profiles referred to above). The roll formed steel profile is then rolled to a radius approximating that of the helically wound plastics body. Finally, the profiled and radiused reinforcing member or members are wound on to the outside of the plastics pipe to form a composite pipe of the requisite stiffness.
When using the reinforcing members disclosed in Australian Patent Nos 607431 and 661047, the step of rolling the steel reinforcing member to a radius of approximating that of the plastics pipe involves straining the steel reinforcing member beyond its elastic limit. This requires the application of considerable force during the rolling process. In contrast, winding of the extruded plastics profile into a helical pipe generally requires much less force due to the material properties of the plastics.
An object of the present invention is to provide certain improvements, beyond those disclosed in the aforesaid Patent Specifications 607431 and 661047, to reinforced helically wound plastics tube or pipe, which are effective to stiffen the tube or pipe through the addition of reinforcing members formed of plastics material and/or of metal to thereby produce a composite plastics and metal structure.
It is another object of the invention to provide an improved composite strip that can be wound formable into a helical pipe or tube without the need for the addition of a reinforcing member during or after the pipe winding process. It is also an object of the invention to provide a method of producing such a composite strip.
It is yet another object of the present invention to provide a method of producing a helically wound pipe that obviates the need for pre-rolling the reinforcing member before it is introduced into the plastics body of the strip.
It is yet another object of the present invention to provide a helically wound plastics pipe formed of two or more materials having different characteristics so that the resultant or finished pipe or tube has enhanced properties, and which can be produced at relatively low cost.
It is yet another object of the present invention to provide a helically wound plastic pipe with a high pressure rating, that is, a pipe that is capable of withstanding high internal pressure without failure.